This invention relates to a burner for the production of a metal oxide article from a volatile compound containing a constituent of the oxide. More particularly, this invention relates to a ribbon burner capable of forming a linear deposit of such an oxide.
The formation of articles such as crucibles, tubing, optical waveguides and the like by depositing metal oxides produced by the flame hydrolysis process upon substrates or mandrels is well known. This process generally involves the vapor phase hydrolysis of volatile anhydrous chlorides of metallic elements from Groups III and IV of the Periodic System, such as for example, silicon tetrachloride, germanium tetrachloride, titanium tetrachloride and aluminum trichloride. U.S. Pat. Nos. 2,272,342; 3,609,829 and 3,737,392 teach methods of making various articles by employing this process. Generally, a single burner has been employed to deposit an oxide layer upon a mandrel or substrate. To coat a planar rectangular area the single burner has been caused to sequentially scan adjacent linear paths. To cover the outside surface of a cylindrical mandrel, the mandrel has been translated and rotated with respect to the flame from the burner as taught in U.S. Pat. No. 3,609,829. This latter mentioned patent teaches a burner for depositing pure fused silica (SiO.sub.2). A stream of SiCl.sub.4 issues from a centrally located aperture on a flat burner face. An annular slot surrounding the central aperture provides a stream of a dry, nonreactive gas such as oxygen or air. By "nonreactive gas" is meant one which does not react with the metal halide vapor at the temperatures at which the gas and vapor emanate from their orifices. If the nonreactive gas is oxygen, for example, it will react with the vapor in the high temperature reaction region which is remote from the burner face. An annular ring of apertures concentric with both the central aperture and the annular slot provides a stream of combustible gases. Since the oxygen issuing from the annular slot does not initially react with the SiCl.sub.4, the SiCl.sub.4 is not decomposed immediately adjacent the burner face, thereby preventing the accumulation of deposited obstructions in or around the apertures of the burner. Such an accumulation or buildup of metal oxide on the burner face is objectionable in that it can obstruct burner orifices, thereby resulting in a nonuniform deposition. Moreover, portions of the buildup can dislodge and become deposited on the target, thereby forming inclusions in the resultant article. The gaseous silicon tetrachloride does intermix and react with the oxygen and fuel a distance from the burner face, and the resultant reaction produces pure silicon dioxide soot which deposits upon the mandrel.
For many applications it is desirable to generate a uniform soot pattern with minimal movement between the work and the soot generating system. It may be desirable, for example, to reduce striations caused by the employment of a single burner to scan the entire surface of the work. It may also be desirable to deposit a greater amount of soot per unit time than that which can be achieved by a single burner. A ribbon or strip burner appears to be the simplest soot generating system for providing large area coverage of the work.
An attempt was made to form a ribbon burner comprising a linear array of SiCl.sub.4 delivery apertures, each surrounded by a narrow slot for providing that aperture with a sheath of shield oxygen, a linear array of fuel-supplying apertures being disposed on opposite sides of the array of annular slots. A burner of this type is disclosed in U.S. Pat. No. 3,565,346. Since each annular slot is formed by a tube disposed within an annular opening in the burner face, the tube cannot be secured to the burner face but must be secured only at that end thereof remote from the burner face. Since it is impossible to maintain these tubes in a parallel array, the streams of SiO.sub.2 particles generated by the reaction of the SiCl.sub.4 gas issuing from each of the central apertures were directed to the mandrel in a nonparallel manner so that the thickness of the deposited SiO.sub.2 coating was nonuniform. Placing slotted supports within the apertures to maintain the tubes in a parallel array obstructed the flow of oxygen from the slots, thereby resulting in a buildup of SiO.sub.2 on the burner face. It was noted that at low flows of inner shield oxygen, a well developed sheet-like stream of SiO.sub.2 soot could be obtained. However, at such low flow rates of inner shield oxygen, a considerable amount of soot buildup occurred on the burner face between the inner shield tubes and the fuel apertures. If the inner shield flow was increased, this buildup was minimized, but the sheet-like nature of the soot stream was adversely affected. At these higher flows, the soot stream consisted of a series of closely spaced discrete streams rather than a continuous sheet.
To obtain a soot deposition of more uniform thickness the inner shield slots were replaced by two linear arrays of orifices, one on each side of the array of SiCl.sub.4 vapor orifices and closely spaced with respect thereto. Two rows of gas-oxygen orifices are located outside the rows of shield gas orifices and are slightly inclined so that the flames therefrom converge a small distance from the face of the burner to establish a reaction zone. The inner shield oxygen is intended to impart a separation between the SiCl.sub.4 vapor and the gas-oxygen reaction products. However, the relatively high velocity flow of oxygen from the inner shield orifices creates a low pressure area which draws a portion of the flame back toward the burner face and the vapor orifices, an occurrence referred to herein as back-splashing. There is also a backward flow of soot particles along the interior surface of the gas-oxygen flames which causes a deposition on the cool face of the burner. Variation of the inner shield oxygen flow does not improve the situation.